Can You Make Up For Different Gear Ratio With a Different Pitch Prop?

[MaxGlide]

To start, I don’t think so. I need a leg that has a gear ratio of 1.97. Fella with a 1.5 ratio leg says “you can just use a different pitch prop”. Doesn’t seem right to me?

 

[Scott06]

To start, I don’t think so. I need a leg that has a gear ratio of 1.97. Fella with a 1.5 ratio leg says “you can just use a different pitch prop”. Doesn’t seem right to me?

Yes and no .

Yes you can prop around some drive ratio difference, just not all drive ratio differences.

I repowered a 4.3 to a 5.0, by the book I should have replaced the 1.81 drive with a 1.62 ratio. I kept the 1.81 and went up in pitch from a 19" to 21 works perfectly hit 4900 rpm at WOT with great hole shot. So it would appear that each drive ratio step is equivalent to 2" prop pitch change.

But going from the steepest gear to the highest gear offered, I'd be loath to think you can just change 6" in prop pitch and have it work, or be able to find a prop that pitch....

I would think if you find a V6 ratio (1.84/1.81) and drop 2" in pitch that would work- say if you run a 19" now drop to a 17"... I just dont think you can prop around three drive ratios steps...

 

[jimmbo]

As Scott06 said, it is a yes and no answer.
While dropping pitch enough will bring up the Rpms, it does result in a loss of Efficiency. There are reasons that the Engine Manufacturers use different Ratios on different Engines, one of which is to reduce the number of Prop designs/pitches they need to produce. This also allows boats to use similar Pitches on a boat, regardless of the HP. As props drop in Pitch, many lines increase Diameter, which is great if you a pushing a Barge, but not good if you are pushing a light Runabout. A fast spinning prop is also far more like to suffer from Cavitation, and can be seen in many underwater images/videos where a Vapour Trail can be seen spiraling behind the Prop Blades. Higher Pitches, along with lower Rpm reduce that. The Image is of an early OMC V4 gearcase, that had a 23:20 ratio, it spun the Prop very fast, and combined with the poorly designed Props OMC had, the Cavitation formed by the Blades is visible. Those V4 had props that were smaller in Diameter than those used on the 28, 33, and 40hp of the era.

As said, you can drop the Pitch enough, but it will not be the same as using the correct Gearing

 

[Alumarine]

With your 140 it would be difficult to find a prop small enough to work with the 1.50 drive.

You should put your engine and boat info in your signature.

 

[MaxGlide]

Yes and no .

Yes you can prop around some drive ratio difference, just not all drive ratio differences.

I repowered a 4.3 to a 5.0, by the book I should have replaced the 1.81 drive with a 1.62 ratio. I kept the 1.81 and went up in pitch from a 19" to 21 works perfectly hit 4900 rpm at WOT with great hole shot. So it would appear that each drive ratio step is equivalent to 2" prop pitch change.

But going from the steepest gear to the highest gear offered, I'd be loath to think you can just change 6" in prop pitch and have it work, or be able to find a prop that pitch....

I would think if you find a V6 ratio (1.84/1.81) and drop 2" in pitch that would work- say if you run a 19" now drop to a 17"... I just dont think you can prop around three drive ratios steps...

So “proper” gear ratio for my setup is 1.97. One I am looking at is 1.65. I will wait to find right leg!

 

[MaxGlide]

With your 140 it would be difficult to find a prop small enough to work with the 1.50 drive.

You should put your engine and boat info in your signature.

Done!

 

[Bondo]

Done!

Ayuh,..... The 1.50, or the 1.65,..??

I think yer gonna find, that with yer little 4 banger,....
They don't make props for the Alphas that small,..... 15" is the smallest,...

Been there, Tried that,.... Years ago,.....
That's the reason I stuffed the 4.3 into the ole fishin' barge,....

 

[MaxGlide]

Ayuh,..... The 1.50, or the 1.65,..??

I think yer gonna find, that with yer little 4 banger,....
They don't make props for the Alphas that small,..... 15" is the smallest,...

Been there, Tried that,.... Years ago,.....
That's the reason I stuffed the 4.3 into the ole fishin' barge,....

He thinks it had the Merc 160. In there so prolly the 1.65 pitch. Whatever it is, I should be able to find the correct leg. There is a group with my exact boat and many are removing the inboard and putting pods on. Just put a wanted post in there. Not really rushed…

 

[Bondo]

He thinks it had the Merc 160. In there so prolly the 1.65 pitch. Whatever it is, I should be able to find the correct leg. There is a group with my exact boat and many are removing the inboard and putting pods on. Just put a wanted post in there. Not really rushed…

Ayuh,...... If yer Lucky,..... it might be a 1.84, which you should be able to prop down too,....
What pitch was on the 1.98,..??

 

[MaxGlide]

Ayuh,...... If yer Lucky,..... it might be a 1.84, which you should be able to prop down too,....
What pitch was on the 1.98,..??

I ended finding a real nice MC-1 with the right ratio, 1.97

 

[jimmbo]

 

[MaxGlide]

Just servicing the leg now. It's in better shape than the one I had on there! Just flushing it, replacing impeller, water pump gaskets, mini o-ring, input U-joint o-rings and pressure testing. Got it almost free!